1. Field of the Invention
The present invention relates to a radiation imaging element, and specifically, relates to a radiation imaging element that converts an amount of radiation transmitted through a subject directly into an electrical signal by use of a radiation sensor.
2. Description of the Related Art
An imaging apparatus that uses a radiation imaging element is used in broad fields including the medical field, the industrial field and the atomic energy field. In a radiation imaging apparatus, radiation is irradiated to a subject, and an intensity of radiation transmitted through the subject is detected to obtain information about the interior of the subject. The radiation imaging apparatus is broadly classified into a direct type imaging apparatus and an indirect type imaging apparatus. In the direct type imaging apparatus, radiation transmitted through a subject is converted directly into an electrical signal to be externally extracted, and, in the indirect type imaging apparatus, radiation transmitted through a subject is first made to be incident on a phosphor and converted into visual light, and thereafter the visual light is converted into an electrical signal to be externally extracted.
In a radiation imaging element used in the direct type imaging apparatus, incident radiation (such as X-rays) is converted directly into an electrical signal (electric charge) by means of an a-Se-based semiconductor layer sensitive to the radiation. For instance, FIG. 5 of Japanese Patent Application Laid-Open (JP-A) No. 2005-101193 is a schematic sectional diagram showing a fundamental configuration of a direct conversion type radiation sensor. The radiation sensor includes:
an active matrix substrate 51, which has multiple collecting electrodes (not shown in the figure) formed in a two-dimensional matrix arrangement disposed in an effective area SA for detection of radiation on a surface of the substrate, and an electric circuit (not shown in the figure) for storage/readout of the electric charges which are collected at the respective collecting electrodes in response to the incidence of radiation;an a-Se-based semiconductor layer 52, which is sensitive to radiation, and is laminated on a side of the active matrix substrate 51 having the collecting electrodes; anda common electrode 53 for applying a bias voltage laminated widely in a planar form on a front side of the a-Se-based semiconductor layer 52.
When a bias voltage is applied from a bias feed power source to the common electrode 53, in a state of applying the bias voltage to the common electrode, electric charges are generated by the radiation-sensitive a-Se-based semiconductor layer 52 corresponding to an incidence of radiation transmitted through a subject to be detected, and collected by the respective collecting electrodes, and then the electric charges are extracted as a radiation detection signal for each of the collecting electrodes by an electric circuit for storage/readout, comprising a capacitor, a switching element, electric wiring, and the like.
That is, in the case of a direct conversion type radiation sensor, respective collecting electrodes arranged in a two-dimensional matrix are electrodes (pixel electrodes) respectively corresponding to respective pixels of a radiation image, and, thereby, radiation detection signals capable of forming a radiation image in accordance with a two-dimensional intensity distribution of radiation projected on an effective area SA for detection of radiation can be taken out.
However, a direct conversion type radiation imaging element involves a large amount of noise, and therefore, an exposure amount of radiation has to be increased in order to obtain a high quality image, which is a problem. Therefore, an improvement of the direct conversion type radiation sensor is demanded.
On the other hand, to make the element thinner, lighter, and more resistant to breakage, attempts are being made to use a resin substrate, which is light in weight and flexible, instead of a glass substrate.
However, fabrication of the transistors using thin films of silicon described above requires a thermal treatment process at a relatively high temperature, and it is difficult to form the transistors directly on a resin substrate which is generally low in heat resistance.
Hence, such TFTs have been actively developed using, as a semiconductor thin film, a film of an amorphous oxide, such as an In—Ga—Zn—O-based amorphous oxide, which can be formed at a low temperature (see, for example, JP-A No. 2006-165529 and IDW/AD'05, pages 845-846 (Dec. 6, 2005)).
As the films for a TFT made with an amorphous oxide semiconductor can be formed at room temperature, the TFT can be prepared on a film (flexible substrate). Therefore, amorphous oxide semiconductors have been attracting attention as a material for active layers of film (flexible) TFTs lately. Particularly, Prof. Hosono et al. of the Tokyo Institute of Technology have reported that a TFT formed using a-IGZO has a field effect mobility of about 10 cm2/Vs even on a PEN substrate, which is higher than that of an a-Si TFT on glass. Since then, TFTs formed using an amorphous oxide semiconductor have particularly drawn attention, especially as film TFTs (see for example, NATURE, vol. 432, pages 488-492, Nov. 25, 2004).
However, in the case of using a TFT formed using a-IGZO, as, for example, a drive circuit of a display, there are problems in that mobility ranges from 1 cm2/Vs to 10 cm2/Vs, which provides insufficient performance, the OFF current is high, and the ON-OFF ratio is low. Particularly, in order to apply such a TFT to a display incorporating an organic EL element, further increase in mobility and improvement in ON-OFF ratio are required.